Field of the Invention
The invention concerns a pulse-width modulator for controlling a semiconductor circuit-breaker in a switched-mode power supply. The pulse-width modulator contains a comparator circuit for generating control pulses determined by an oscillator signal, whereby a duration of the individual control pulses depends on a first control signal and a second control signal. A measuring configuration is provided for generating a load-current signal dependent on the load current of the circuit-breaker. A feedback branch for feedback of the load-current signal to the comparator circuit is also provided. The feedback branch has a low-pass configuration with an input that receives the load-current signal and an output at which the second control signal can be picked off.
Pulse-width modulators of this kind serve for controlling the output voltage or the output power of a switched-mode power supply.
A pulse-width modulator with the above features is known, for example, from U.S. Pat. No. 5,390,101.
Furthermore, Published, European Patent Application EP 0 584 623 A1 discloses a converter for generating a constant output voltage with a current-mode regulator, which converter has a post-connected R-C element for achieving freedom from peaks whereby the peaks always arise at the beginning of the switch-on phase of the circuit-breaker.
The power input or output of the switched-mode power supply depends among other things on the duration of the periodically generated control pulses, which effect closure of the semiconductor circuit-breaker for the duration of the control pulse and therewith cause a load current to flow. Control of the duration of the control pulses, which are normally generated at the clock frequency of the oscillator signal, takes place in the circuit configuration in dependence on the first and a second control signals and whereby the first control signal depends among other things on the output voltage or output power of the switched-mode power supply.
Thus in the prior art pulse-width modulators for controlling the semiconductor circuit-breaker closure of the semiconductor circuit-breaker takes place at the clock frequency of the oscillator signal, whereby the semiconductor circuit-breaker is opened again in dependence on the sequence of the first and second control signals. Normally the control pulses are selected such that they end when the second control signal exceeds the first control signal, as a result of which the semiconductor circuit-breaker is opened.
In prior art pulse-width modulators of this kind the load-current signal is fed back to the comparator circuit either directly or through an amplifier. As a result of unavoidable parasitic capacitances, a short, high switch-on current pulse arises in the load current flowing in the switched-mode power supply after the semiconductor circuit-breaker is switched on, which if fed back directly into the comparator circuit as a second control signal would result in the semiconductor circuit-breaker being switched off immediately after being switched on if the switch-on pulse exceeds the first control signal. These kinds of pulse-width modulators for controlling the semiconductor circuit-breaker therefore have configurations in the feedback branch which blank out the load-current signal at the beginning for a period which is longer than the duration of the switch-on pulses.
A disadvantage of these kinds of pulse-width modulator is that it is not possible to generate any control pulses with a duration shorter than the period during which the load-current signal is blanked out. Because the power output of the switched-mode power supply depends on the duration of the control pulses, there is a lower limit to output power of a switched-mode power supply with a pulse-width modulator of this kind.